Recently much work has been done in the field of inverter-induction machine systems. It has been found that a conventional inverter circuit can be connected to operate not as an inverter, but to recirculate the reactive energy from an induction machine driven as a generator, thus replacing the conventional capacitor bank or additional machine used for excitation. Such a system is described and claimed in U.S. Pat. No. 3,829,758, entitled "AC-DC Generating System", which issued Aug. 13, 1974 to George H. Studtmann, and is assigned to the assignee of this invention. Subsequent work determined that by modulating the switching frequency of the inverter circuit (which is recirculating the reactive energy) to switch above and below the synchronous frequency of the machine itself, an alternating output voltage can be produced. Such an arrangement is described and claimed in the application of George H. Studtmann and Hary J. Venema, entitled "Modulated Induction Generator", filed Apr. 16, 1975, Ser. No. 568,746, and assigned to the assignee of this invention. Additional research effort determined that the induction machine when operated as a generator in the modulated induction generator system was in effect derated or utilized substantially below its nominal rating. It was found that the efficiency of the induction machine, and thus of the overall system, could be substantially enhanced by utilizing a bipolar inverter in the system, in place of a conventional converter. A bipolar inverter is one which provides alternating energy to the load connections, irrespective of the polarity of the energizing potential difference supplied to the inverter. Such use of a bipolar inverter with an induction machine is described and claimed in the copending application of George H. Studtmann, entitled "Bipolar Inverter with Induction Generator", filed June 27, 1975, Ser. No. 590,883, and assigned to the assignee of this invention. That application further explains that the bipolar inverter has substantial utility, not only in connection with an induction generator system, but also in a system which regenerates or returns power from a load through the inverter toward the source, and in an ac-to-ac or frequency heterodyne conversion system.
One disadvantage of the bipolar inverter system described and claimed in that application is that commutation for the thyristors in the power switches is provided by an auxiliary (aux-comm) circuit, and must be separately triggered on to effect commutation of the then-conducting thyristor.
It is therefore a primary object of this invention to provide a bipolar inverter or power conversion circuit in which the commutation for the thyristors is provided by a complementary commutation circuit.
A corresponding object of the invention is to provide such a complementary commutation circuit, with a reduced number of thyristors as contrasted to auxiliary commutation circuits, and a simplified logic circuit for turn-on of the thyristors as contrastedd to the control arrangements for the aux-comm circuits.